1. Field of the Invention
The present invention relates in general to a joined metal-ceramic assembly or structure, and more particularly to a metal-ceramic assembly obtained by brazing a metal member and a ceramic member such that the metal and ceramic members are butted together with an intermediate layer interposed therebetween. The invention is also concerned with a method of preparing such a joined metal-ceramic assembly.
2. Discussion of the Prior Art
In recent years, ceramic materials have been increasingly evaluated by their excellent properties and utilized for various industrial applications, for example, as piston caps, turbocharger rotors and gas turbine rotors. To keep pace with the increasing utilization of ceramic materials, various methods of joining together a ceramic member and a metallic member have been proposed.
According to one of the proposed joining methods, a separate intermediate layer is interposed between the joining surfaces of a ceramic member and a metallic member when these two members are butted together For example, laid-open Publication No. 59-160533 of unexamined Japanese Utility Model Application discloses a method wherein a joined ceramic-metal structure is prepared by joining a ceramic member and a metallic member with a tungsten (W) or molybdenum (Mo) clad member interposed between the joining surfaces of the ceramic and metallic members. Another method is disclosed in laid-open Publication No. 60-255679 of unexamined Japanese Patent Application, in which an intermediate brazing metal layer interposed between a ceramic and a metallic member is machined to remove a portion of the brazing metal layer so that the brazing metal does not contact the ceramic and metallic members, except the joining surfaces. A further method is proposed in laid-open Publication No. 62-6773 of unexamined Japanese Patent Application, in which a ceramic shaft and a metallic shaft are joined together such that a short intermediate metal shaft having a lower Young's modulus than the ceramic and metallic shafts is interposed therebetween.
However, the above-indicated joining methods wherein the ceramic and metallic members are butted together suffer from a problem that the prepared ceramic-metal structure has a residual stress at the joining surfaces of the ceramic and metallic members due to a difference in coefficient of thermal expansion, and therefore, the structure does not have sufficient joining strength.
Described more specifically, the metallic and ceramic members are butted together while they are heated. when the joined metal-ceramic structure, which assumes a nominal shape at the elevated joining temperature, is cooled, the metallic member having a higher coefficient of thermal expansion than the ceramic member contracts more than the ceramic member, causing tensile stresses to be exerted on the peripheral portion of the joining interface of the two members. Such stresses may result in separation or cracking of the two members, which originates at the periphery of the joint.
While various types of intermediate layers to be interposed between the ceramic and metallic members have been proposed, the problem addressed above cannot be sufficiently solved, since the intermediate layer is not used for the purpose of eliminating the residual stresses at the peripheral portion of the joining surfaces. Namely, joined metal-ceramic assemblies or structures produced according to the known methods still suffer from residual stress concentration on the peripheral portion of the joined end of the ceramic member, which considerably reduces the joining strength. Thus, the difference in coefficient of thermal expansion between the ceramic and metallic members causes an undesirable residual stress at the joined end of the ceramic member, whereby the obtained metal-ceramic assembly is not provided with a sufficiently high degree of bonding strength.